Magnetic colloids are liquids having magnetic properties in which ferromagnetic materials are collodially suspended. Such ferrofluids or magnetic liquids must show a high degree of stability (e.g., to gravitational and magnetic fields) in order to perform well in various commercial devices and be responsive to external magnetic fields. Generally a stable magnetic colloid or ferrofluid in a high magnetic field gradient requires small ferromagnetic particles of generally less than 200 angstroms in diameter. The ferromagnetic particles are usually coated with one of several layers of surfactants to prevent agglomeration.
Typical ferrofluid compositions are described, for example, in U.S. Pat. No. 3,700,595, (1972), wherein anionic surfactants, such as fatty acids, alcohols, amines or amides and other organic acids are employed as dispersing surface active agents; U.S. Pat. No. 3,764,504, (1973), wherein aliphatic monocarboxylic acids are employed as dispersing agents; U.S. Pat. No. 4,208,294, (1980), wherein a water-based magnetic liquid is produced using C.sub.10 to C.sub.11 aliphatic monocarboxylic acids as acid dispersing agents; and U.S. Pat. No. 4,430,239, (1984), wherein a stable ferrofluid composition is provided which employs a phosphoric acid ester of a long-chain alcohol as a surfactant.
Various processes have been described for preparing magnetic colloids and ferrofluids. For example, U S. Pat. No 3,917,538, (1975) describes a process for preparing an irreversibly flocked magnetic particle using different dispersing agents, such as nonionic and anionic surfactants, and wherein the ferrofluids are prepared employing a grinding or ball mill technique; U.S. Pat. No. 4,019,994, (1977) describes a petroleum sulfonate surfactant with an aqueous carrier; U.S Pat. No. 4,356,098, (1982) describes ferrofluid compositions composed of a silicone-oil carrier and a dispersing amount of an anionic surfactant which forms a chemical bond with the surface of the magnetic particles as a tail group compatible with or soluble in the silicon-oil carrier; and U.S. Pat. No. 4,485,024, (1984), describes a ferrofluid produced by controlling the pH of an aqueous suspension of the ferromagnetic particles of an organic solvent together with surface active agents, such as fatty carboxylic acids.
A properly stabilized ferrofluid composition typically undergoes practically no aging or separation, remains liquid in a magnetic field and after removing of the magnetic field shows no hysteresis. A stabilized ferrofluid exhibits stability by overcoming generally three principal attractive forces; van der Waals, interparticle-magnetics and gravitational forces.
In the computer industry, static charge build up of the disk in a disk drive occurs due to its rotation and needs to be grounded. In addition, the disk cavity must be hermetically sealed for contamination-free operation. Electrically conductive ferrofluids which contain finely divided dispersed carbon particles are useful for this purpose however, there is a need to restrict the amount of carbon black employed in the ferrofluid compositions to avoid gradual increases in the viscosity of the composition and absorption of the fluid into the carbon particles with time. The addition of carbon black to a typical ferrofluid composition provides a composition which tends to be pseudoplastic when amounts greater than about five (5%) percent of carbon black are used, while a low concentration of carbon black (e.g., less than 5%) provides a Newtonian, non-conductive composition. A stable, low viscosity, highly electrically conductive ferrofluid composition is needed, with or without carbon black, particularly for use in computer seals, as well as for other devices where a stable, low viscosity, highly electrically conductive ferrofluid composition is required.